1. Field of The Invention
The invention relates to a pulse width modulated solenoid valve for control of hydrodynamic torque converters of automatic transmissions.
2. Description of the Prior Art
Automatic transmissions of motor vehicles are generally equipped with hydrodynamic torque converters such as are described, for example, in EP-A 0 433 619 or EP-A 0 419 782, or in the book by Gerigk, Bruhn & Danner, "Kraftfahrzeugtechnik", Westerman-Verlag, 2nd edition, pages 349-351. Hydrodynamic torque converters enable the vehicle to move off gently and smoothly at low rotational speeds of the output shaft of the engine and facilitate smooth gear-shifting of the automatic transmission. The use of hydrodynamic torque converters results in a low-noise, low-wear, and stepless transfer of the engine torque to the transmission.
Hydrodynamic torque converters comprise an impeller connected to the engine output shaft, a stator supported through a freewheel, and a turbine wheel connected to the shaft leading to the gearbox. The housing of the torque converter, in which these parts are arranged, is completely filled through a system of oil lines with a pressure fluid, which in the case of motor vehicles is usually hydraulic oil. The pressure in the hydrodynamic torque converter is regulated by means of control valves. The invention will be explained hereinafter with reference to hydraulic oil, though it will be understood that it is in no way limited to the use of hydraulic oil as pressure medium.
At low input rotational speeds, the impeller transfers part of the kinetic energy to the hydraulic oil, which at the same time sets the turbine wheel in motion. By means of the stator, the oil flow is diverted so as to amplify the action of the impeller. This state of the torque converter is known as "unlock".
At low rotational speeds, a clutch unit arranged between the impeller and connected to the drive and to the turbine wheel connects the impeller positively to the turbine wheel. This state is known as "lock-up".
Through the positive transfer of the torque in the "lock-up" state, any unsteady behavior of the drive is transmitted directly through the torque converter to the transmission, thereby adversely affecting the driving behavior of the vehicle.
To ensure that the torque is also transferred smoothly in the "lock-up" stated particularly during gear-shifting, the slip between the impeller and the turbine wheel can be varied. By means of control valves, the pressure of the hydraulic fluid within the torque converter is changed so that the clutch device is opened for short periods. The clutch then slips, that is to say, the slip between the impeller and the turbine wheel increases and abrupt, short-term changes in torque due to unsteady drive behavior can thus be compensated for.
For control of the clutch device, the hydrodynamic torque converter has two inlets controlled by means of a control valve which are provided with hydraulic oil under pressure according to the state of engagement of the clutch device. The first of the two inlets is situated directly on the converter housing and acts on the engaging side of the clutch, while the second inlet is connected to the disengaging side of the clutch device.
At low speeds of rotation of the turbine wheel, a lower pressure is applied at the first inlet of the clutch device, while the second inlet is subjected to a markedly higher pressure. The pressure difference between the inlets causes the clutch device to move away and thereby to disengage. The hydraulic oil flowing through the second inlet into the clutch device flows through the clutch device into the converter housing and thence via the first inlet into the control hydraulic circuit (unlock).
On reaching the point of shift between unlock and lock-up, the first inlet is subjected to a markedly higher pressure than the second inlet. The pressure now acting in the converter housing presses the coupling device together, so that the impeller is positively connected to the turbine wheel (lock-up).
As mentioned above, in order to regulate the slip between the impeller and the turbine wheel during the lock-up, the pressure acting at the first inlet of the converter is varied by means of the control valve.
Since the hydraulic oil is subjected to churning work through the continual movement, the hydraulic oil must constantly be cooled by means of a cooling device located outside the converter. The diversion of the hydraulic oil to the cooler is likewise regulated by means of a control valve, which is often the same one as regulates the flow of the hydraulic oil to the converter.
If the hydrodynamic torque converter is shifted in the "lock-up" state, the oil passages to the torque converter and the line leading to the hydraulic oil cooler are switched over by means of the control valve at the same time.
The control of the flow of the hydraulic oil, which serves in hydrodynamic torque converters to drive the turbine wheel and impeller and to shift the clutches integrated in the torque converters, is performed, as mentioned above, by means of control valves, which are mostly pulse width modulated solenoid valves.
Pulse width modulated solenoid valves for hydrodynamic torque converters are well known. They comprise an electromagnetic switching element and a control valve with a control spool arranged slidably therein which closes and/or opens the various inlets and outlets of the control valve. At the end of the valve body facing the electromagnetic switching element, there is a control pressure chamber with several inlets and outlets. At one of the inlets, the control pressure connection, which is opened and closed by the electromagnetic switching element, a substantially constant control pressure of, for example, 8 bar, is applied. On operation of the electrical switching element, the control pressure connection is opened and the control pressure acting on the control pressure connection displaces the control spool, thereby opening a converter clutch control connection connected to the control pressure chamber. Through this converter clutch control connection, the hydraulic oil flows into the converter housing. Depending on the duration of opening of the control pressure connection, the pressure acting directly at the outlet of the switching element in the control pressure chamber is between 0 bar and the maximum control pressure.
The electrical switching element is operated at a predetermined switching frequency, for example, 40 Hz, while the duration of opening of the control pressure connection per switching pulse, the so-called pulse width, can be varied. By varying the pulse width, the control pressure acting on the clutch unit can be regulated between 0 bar and the maximum value so that the slip between impeller and turbine wheel can be influenced. If the control pressure connection remains open over the whole pulse width, the maximum control pressure acts and the clutch remains closed, while in case of a shorter duration of opening, the control pressure in the converter housing is correspondingly decreased.
The rapid opening of the control pressure connection for short periods of time gives rise to pressure spikes which are propagated through the converter clutch control pressure connection into the converter housing. This leads, on switching of the clutch, to fluctuations in the torque, influenced by the pressure spikes, and to unsteady driving behavior connected therewith.
The pulse width modulated solenoid valves hitherto used in the control unit were not provided with a spring to achieve a "soft" end position at the end of the valve housing remote from the switching element.
Although these solenoid valves operate satisfactorily, they nevertheless lead at all pulse widths to the pressure spikes being passed on and to the torque fluctuations in the drive associated therewith.